Redox species have inherent electrochemical activity and are therefore capable of exchanging electrons directly with a working electrode to produce an electrochemical signal. This allows direct electrochemical detection and quantfication without a requirement for a mediator means, such as via an enzyme mediated reaction.
The quantification of redox species in the prior art has had to rely on laboratory based analytical techniques such as High Pressure Liquid Chromatography (HPLC) attached to a detector comprising a series of porous electrodes at different potentials (CoulArray™). While this system is suitably accurate, the size and expense of the equipment render it unsuitable for large scale sample testing or in-home consumer usage. Furthermore, biological samples must commonly undergo extensive pre-treatment to provide them in a form that can be analysed by HPLC.
With these matters in mind it would be dearly desirable to develop a means of quantifying redox species which is not only accurate, but is also conveniently portable, disposable and able to analyse a biological sample which has undergone minimal pre-treatment.
Portable disposable biosensors of the prior art are not capable of accurately quantifying redox species at their naturally occurring levels in a biological sample. The redox species in enzyme mediated biosensors, such as ferricyanide ions released in standard glucose biosensors, are produced in abundance and easily quantifiable. In contrast the concentration of any individual redox species naturally present in a biological sample is comparatively low, making the accurate quantification of the electrochemical signal due to any individual spedes by conventional biosensors impossible.
The concentrations at which redox species are present in biological samples also renders background electrochemical activity from interferents, such as proteins, a particular problem. The magnitude of the electrochemical signal from the interferent commonly masks or skews the electrochemical signal of the redox species for which detection is sought. Conventional biosensors have no means of adequately making the distinction between interferent electrochemical activity and that of a particular redox species under analysis.
The present invention therefore aims to provide a disposable electrochemical biosensor which can accurately quantify redox species in a biological liquid sample.
A number of redox sensors are known which utilise an enzyme mediator to generate the redox species to be detected by enzymatic reaction with a component in the sample. These are disclosed in U.S. Pat. No. 4,795,452, GB-A-1 554 292, U.S. Pat. No. 5,746,898 and WO-A-93/13408. The latter document discloses a system in which a liquid/solid interface is provided with the electrodes by virtue of a liquid sample filling a well. U.S. Pat. No. 3,847,777 discloses a gas sensor in which a wet electrolyte system is used to interface with the electrodes. For simple and cost effective manufacture of an electrochemical system usable in the home, there is a need to provide a system which does not rely on an enzymatic mediator or a wet electrode arrangement. However, such a system would still need a filter to reduce interferents yet be capable of wetting the electrodes.